Around midday on Monday, Congressman Al Green, a Democrat from Texas, held a press conference to call for the impeachment of Donald Trump. The firing of FBI director James Comey, Green said, was an obstruction of justice falling clearly into that basket of “high crimes and misdemeanors” prescribed in the constitution as grounds for impeachment.

Green should have waited a day. Because by the time the sun went down on Tuesday , advocates for Trump’s impeachment had a lot more to work with.

On Tuesday evening, the New York Times reported that Trump asked former FBI director James Comey to drop an investigation into former national security adviser Michael Flynn. “I hope you can see your way clear to letting this go, to letting Flynn go,” Trump said to Comey, according to Comey’s notes on the meeting. “He is a good guy. I hope you can let this go.”

The White House denied the report, which was subsequently confirmed by numerous media outlets.

The Comey news broke before Washington had a chance to catch its breath from Monday’s shocking revelation. Late on Monday afternoon, the Washington Post reported that Trump had divulged highly classified material to Russian diplomats in an Oval Office meeting last week – material so sensitive that homeland security officials scrambled to place calls to US intelligence agencies afterwards to warn them that the information had leaked, via the president’s mouth, to Moscow.

Trump told Russian foreign minister Sergei Lavrov and ambassador Sergey Kislyak about spying by an unnamed US partner that had revealed an alleged Islamic State plot involving laptop computers and airplanes, the Post reported.

“As president I wanted to share with Russia (at an openly scheduled WH meeting) which I have the absolute right to do, facts pertaining … to terrorism and airline flight safety,” Trump wrote. “Humanitarian reasons, plus I want Russia to greatly step up their fight against Isis and terrorism.”

The episodes have once again stoked the chorus calling for the impeachment of Trump, a chorus that has steadily built over the four months of the Trump presidency.

Some Democratic lawmakers immediately questioned the legality of Trump’s alleged statements to Comey, with Senator Dick Durbin saying it “again appears to cross the line into the obstruction of justice”. Elijah Cummings, a top congressional Democrat, said, “clearly we’ve got a smoking gun and a lot of dark smoke.”

As for Monday’s bombshell,legal analysts say that Trump is correct in noting his “absolute right”, as president, to share information as he pleases. The president’s discretion overrides any categorical classification, and there has been no assertion that Trump broke a law by allegedly sharing the information.

The president may, however, have broken his oath of office, according to analysis at Lawfareblog, whose top six analysts joined in a byline to write: “It’s very hard to argue that carelessly giving away highly sensitive material to an adversary foreign power constitutes a faithful execution of the office of president.”

The blog notes that allegations of oath violations have been central to every previous case of impeachment or near impeachment.

“Let’s not minimize it,” legal scholar Alan Dershowitz said on CNN late Monday. “Comey is in the wastebasket of history. Everything else is off the table. This is the most serious charge ever made against a sitting president of the United States. Let’s not underestimate it.”

The case against Trump was, to some minds, already strong. A campaign is under way to impeach Trump for allegedly violating constitutional bans on receiving certain gifts. Others have argued for an arcane application of constitutional law under which the vice-president and cabinet together might declare the president unfit to serve.

Green was not alone in seeing the firing of Comey as the last straw. As FBI director, Comey was leading an investigation into alleged ties between the Trump presidential campaign and Russian operatives. Trump told interviewer Lester Holt last week that the Russian investigation was on his mind when he fired Comey.

“And in fact when I decided to just do it, I said to myself, I said ‘you know, this Russia thing with Trump and Russia is a made-up story, it’s an excuse by the Democrats for having lost an election that they should have won’,” Trump said.

To many ears, the lines were a blank admission of obstruction of justice à la Nixon. In a Saturday Night Live parody of the scene, Holt, played by Michael Che, stops the interview to ask his production team: “So … did I get him? Is this all over?” Holt listens to his earpiece. “No, I didn’t? Nothing matters? Absolutely nothing matters any more?”

Who has been impeached before?

Two presidents, Bill Clinton (1998) and Andrew Johnson (1868). (Congress may also impeach judges.) Articles of impeachment were passed against Richard Nixon by a congressional committee, but Nixon resigned before the House of Representatives could vote on the matter, meaning that technically he was not impeached.

Impeachment does not mean expulsion from office. Under the constitution, impeachment happens in the House of Representatives if a majority approves articles of impeachment previously approved in committee. Then impeachment goes to the Senate, where a two-thirds majority vote is required to convict the president, upon which he would be removed from office.

Both Johnson and Clinton were impeached in the House but then acquitted in the Senate and remained in office.

What can a president be impeached for?

“Treason, bribery, or other high crimes and misdemeanors”, the constitution says. Needless to say, there’s debate over what all those terms mean.

Johnson was charged with breaking the law by removing the US secretary of war, which, in the aftermath of the civil war, was not his decision as president to make. Clinton was charged with obstruction of justice and with perjury, for allegedly lying under oath to a federal grand jury about his affair with Monica Lewinsky.

Had Nixon not resigned, he might have been convicted in the Senate on one of three charges: obstruction of justice, abuse of power or defiance of subpoenas. In any case, Gerald Ford, who was Nixon’s vice-president and who succeeded him, pardoned Nixon of any crimes a month after Nixon resigned.

Can a president be removed apart from through impeachment?

Theoretically, yes, under the aforementioned 25th amendment, which was ratified relatively recently, in 1967, to clear up succession issues made painfully urgent by the assassination of John F Kennedy.

The 25th amendment describes a process by which a president may give away power owing to his or her own disability, and a separate process by which power may be taken from a president owing to disability or inability.

The key players in the second case are the vice-president and the top 15 members of the cabinet. If the former and a majority of the latter decide the president is “unable to discharge the powers and duties of his office”, they submit that information in writing to the House speaker (currently Paul Ryan) and Senate president pro tempore (currently the Utah Republican senator Orrin Hatch) and just like that, the vice-president would be acting president.

The president may challenge such a decision, at which point a two-thirds majority in both chambers of Congress would be required to stop the president from regaining power.

It’s conceivable that Trump would not go quietly if his cabinet and vice-president Mike Pence were to gang up on him.

How long do impeachment proceedings take?

There isn’t much precedent to say, but the Clinton case proceeded through Congress relatively quickly, in about three months. That example may be misleading, however, owing to the years-long investigation of Bill and Hillary Clinton, including the Lewinsky affair, by the special prosecutor Kenneth Starr, which preceded it. Starr handed his report and research to the House judiciary committee, which therefore had no need to conduct a time-consuming investigation of its own.

Richard Nixon waves goodbye from the steps of his helicopter as he leaves the White House following a farewell address to his staff on 9 August 1974. Photograph: Chick Harrity/AP

Who’s saying Trump should be impeached?

About 46% of Americans who responded to a Public Policy Polling survey in February, for starters. Public opinion matters because for impeachment to happen, Congress must act, and elected officials sometimes hang their principles on opinion polls.

It’s notable that Nixon, a Republican, faced impeachment in a Congress controlled by Democrats, and Clinton was impeached by a Republican-controlled Congress. For Trump to be impeached, members of his own party would have to turn on him.

That’s why Republican base approval of Trump is so important. If Republican voters do not abandon the president, Republican members of Congress are not likely to.

On the other hand, the Republican Congress might conceivably be enticed into action by the prospect of dumping Trump in exchange for someone they are far more comfortable with: Pence, himself a former congressman and a much more predictable traditional conservative.

Several congressional Democrats have called for impeachment proceedings of some kind. Representative Mark Pocan of Wisconsin said on the House floor in February that if Trump did not divest business holdings and take other actions, then “we’ll have to take other actions, including legislative directives, resolutions of disapproval and even explore the power of impeachment”.

What might Trump be charged with?

Bonifaz, of the Impeach Trump Now group, argues that Trump’s failure to divest from his businesses has already produced frequent violations of constitutional rules for emoluments, or gifts. Investigations into associations with Russia by Trump or his proxies could conceivably produce some kind of disloyalty charge. If Trump has to testify at some point about any of this, he could face a Clinton-style perjury charge. Abuse of power? Obstruction of justice? It seems as if, should Congress get to the point of charging Trump, they may have a buffet of potential charges to choose from.

What would it take in practice to trigger enough Republicans into action?

Republicans had promised to impeach Hillary Clinton as soon as she took office. But what they might not have remembered is that two-thirds of the Senate is required to convict a president of impeachable offenses. The Democrats are in the minority, but they do have 48 Senate seats out of 100.

The most important factor for Republicans in deciding whether to go after Trump would seem to be the disposition of Republican voters. If the people turn on the president, Congress may follow.

What kind of mood does Pence seem to be in? Is he implicated? In the scenario of a Pence succession, it would be up to the current vice-president to pardon Trump or not. Maybe Trump would be more likely to get out of the way and avoid all or some impeachment proceedings – in this truly hypothetical scenario – if Trump felt reassured that Pence, upon acceding to power, would pardon him.

Could he refuse to comply with proceedings?

Only two months into the Trump presidency, we’ve already heard warnings, issued by members of Congress, about this or that constitutional crisis being afoot: Trump impugns judges; Trump overrides legislated regulations; courts block executive actions. There are many opportunities for further constitutional crises during the Trump years, and a Trump refusal to go along with prospective impeachment proceedings is certainly easy to imagine. In which case: who controls the military?

Could he refuse to step down if he’s found guilty?

See above.

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Would Pence go down with him?

Not likely. There’s a school of thought that says a key reason the Republican congressional majority would assent to a Trump impeachment is because then they would get the president they really want, Pence. The closest historical precedent to a double whammy of this kind is the resignation in a bribery scandal of Nixon’s vice-president, Spiro Agnew, in 1973, a year before Nixon went down. But the alleged crimes were unrelated.

Elections have consequences. Is this all actually just a fantasy for liberals and conservative purists who cannot accept that Trump won?

Reasons this might not be true include the fact of Trump’s historically low popularity rating at the two-month mark. Trump sits at 38% approval, according to Gallup, 20-some points behind the historical average for first-term presidents. Additionally, Trump seems to be flying unusually close to the sun, in terms of his conduct as an elected official. During his campaign he refused to release his tax returns on the grounds that the usual rules did not apply to him. His refusal to divest from his businesses as president, on similar grounds, could lead him into legal hazards that other presidents have avoided. And the investigations into his campaign’s ties to Russia continue. Finally, Trump is truly a Washington outsider, which could increase his vulnerability to acts of bureaucratic infighting or hidden treachery, as evidenced by the incredible number of leaks from the intelligence community so far.

On the other hand, the election of Trump has been a particularly painful blow to the progressive psyche, more so even perhaps than the re-election of Bush as the atrocities of the Iraq war mounted in 2004. Republicans suffered for eight years from what some of their critics called Obama derangement syndrome, becoming so wrapped up in their opposition to the president that any sense of greater purpose seemed sometimes to be lost. Are progressives and conservative purists suffering from Trump derangement syndrome? It’s possible. We’ll find out.